Scientists have long thought that such lunar craters arose during a period called the Late Heavy Bombardment (LHB), about 4 billion years ago. During that time, a hailstorm of giant asteroids pummeled the solar system, slamming into the moon, along with young planets like Mercury, Venus, Earth, and Mars. Evidence for this theoretical period comes mostly from the moon itself. While most traces of Earth's early history have been wiped away by erosion and tectonic activity, the moon remains as a nearby, relatively untouched, and easily observable relic of the early solar system. In particular, scientists have based most of their theories of that period on the impact basins found on the moon's near side — the side always facing the Earth — assuming, from the size of its craters and basins, that the moon and other planets endured impacts from massive asteroids. But now scientists from MIT, the University of Paris, and elsewhere have found that craters on the near side of the moon may not reflect the intensity of asteroid impacts from that period. Instead, much smaller asteroids likely created these craters — a finding that may redefine scientists' picture of the LHB. Video: MIT News Computer simulations: Katarina Miljkovic Additional footage and still images courtesy of NASA

• published: 07 Nov 2013
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